Transform Drawing using GCPs

Rubber sheeting with GCPs given in CSV.

Often, CAD users have a drawing in one coordinate system and want to transform it into another coordinate system. The best and easiest way is to acquire points for which the coordinates are known in both coordinate systems. These points can be called ground control points (GCPs). The following graphic shows three of such points that we can use for transformation.

SPCAD offers a custom transformation tool that receives the GCPs in a CSV file and transforms drawing’s coordinates from one system to the other. SPCAD requires a CSV file formatted with five columns: the first column for the serial number of GCPs, the second and third columns for the x and y coordinates of the source coordinate system, respectively, and the fourth and fifth columns for the x and y coordinates of the target system. Each row represents one GCP.

SPCAD employs a 4-parameter Helmert transformation, which retains the shape of the features while performing translation in the x and y directions, as well as rotation and scaling of the features. A distinct quality of SPCAD is that it displays the residual error in the set of GCPs, which helps users determine the quality of the given GCPs, as shown in the above graphic.

In the following figure we have survey data in the Lambert Conformal conic based coordinate system. and we want it to be in related UTM coordinates system.

SPCAD reads the CSV and computes the residuals.

After the user is satisfied and presses OK, the features are transformed, and a prompt is given to type the name of the transformed drawing, which is then saved in the user-specified folder.

The transformation of drawings implemented by SPCAD is also considered as rubber sheeting. It employs GCPs to spatially adjust and rectify drawings, adjusting translation in both directions, rotation, and scale, similar to rubber sheeting techniques.

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